1. Technical Field of the Invention
The present invention relates to combustion pressure sensors for use in internal combustion engines and, more particularly, to a glow plug with combustion pressure sensor for detecting a pressure in a combustion chamber formed in an engine head to allow an engine to be controlled based on a detected pressure to achieve an optimized combustion state.
2. Description of the Related Art
There has heretofore been generally known a glow plug with combustion pressure sensor composed of a glow plug, preheating a combustion chamber when starting up an engine, and a combustion chamber which are integrally structured for detecting a pressure inside the combustion chamber. Japanese Patent Application Publication No. 2005-90954 discloses one example of such a structure, which is shown in FIG. 3. FIG. 3 is a typical view showing a glow plug with combustion pressure sensor 300 of the related art that is mounted on an engine head 301.
Hereunder, for the sake of convenience of illustration, an upper area and a lower area in FIG. 3 are referred to as a base end or base end portion and leading end or leading end portion, respectively.
The glow plug with combustion pressure sensor 300 has a heating rod 31, having a leading end exposed to a combustion chamber 302, which has a base end portion connected to an intermediate shaft 37 made of metal to act as an electrode. The intermediate shaft 37 and a heating member are electrically connected to each other. The intermediate shaft 37 protrudes from a housing 30 and fixedly retained with a contact tube 34 via an O-ring 38.
With such a structure, the heating rod 31 is displaced toward the base end of the glow plug with combustion pressure sensor 300 in response to fluctuation in combustion pressure inside the combustion chamber 302. This causes the contact tube 34, fixed to the heating rod 31, to be displaced toward the base end. With such displacement, a diaphragm 35, fixed to the engine head 301 via the housing 30, has one portion, fixedly secured to a base end of the contact tube 34, which is displaced toward the base end relative to another portion fixed to the housing 30. This causes strain to occur on the diaphragm 35. A combustion pressure sensor 36, placed on the diaphragm 35 at a base end thereof, detects a pressure inside the combustion chamber 302 based on such strain.
With the structure of the related art shown in FIG. 3, the combustion pressure sensor 36 takes the form of a structure exposed to outside air. With such a structure, the combustion pressure sensor 36 directly receives an effect of outside air prevailing at a base end portion of the cylinder head 301. Thus, the combustion pressure sensor 36 detects the combustion chamber with degraded precision. In particular, with the combustion pressure sensor 36 arranged to detect the combustion pressure based on small changes in strain resulting from fluctuation in combustion pressure, a pyroelectric effect occurs due to moisture contained in outside air. This causes the combustion pressure sensor 36 to generate an output signal with variation caused by the pyroelectric effect, resulting in the detection of the combustion pressure with degraded precision.
With such a structure of the related art set forth above, an attempt may be made to provide a package member to cover the combustion pressure sensor 36. For the combustion pressure sensor 36 to be completely shut off from outside air, the package member and the intermediate shaft 37, made of metal, need to be hermetically sealed by welding. When this takes place, the package member and the intermediate shaft 37 are fixed to each other with accompanying difficulty of causing the heating rod 31 and the contact tube 34 to be displaced in an axial direction. Thus, the combustion pressure sensor 36 cannot take a structure needed for detecting the combustion pressure.
To address such an issue, the package member may be arranged to retain the intermediate shaft 37 via, for instance, an O-ring. Even under such an arrangement, a drag occurs on a contact portion between the package member and the O-ring due to sliding resistance occurring thereon during axial displacement of the intermediate shaft 37. This results in an effect of suppressing displacement of the heating rod 31, causing the combustion pressure sensor 36 to have difficulty in detecting the combustion pressure with high precision. In addition, the O-ring has an area, held in contact with the intermediate shaft 37, which is progressively worn away in operation of the combustion pressure sensor 36. Thus, the O-ring encounters a difficulty of ensuring a hermetic sealing effect, causing the combustion pressure sensor 36 to have a risk with the occurrence of pyroelectric effect.